Decentralized Wastewater Treatment Systems (DEWATS) are increasingly being recognized by decision makers and city developers across the world as an option for service delivery in densely populated low-income areas. However, little knowledge and field-data are available on some key design-, operation- and monitoring-parameters. This paper addresses these gaps by presenting data on per capita wastewater production of communities connected to DEWATS, hydraulic peak flow factors, per capita BOD5 load estimates, biogas-production of DEWATS biogas digesters, DEWATS effluent characteristics and their fluctuation over time and settled reactor sludge characteristics concerning TS and VS concentrations, specific methanogenic activity (SMA), effect of sludge storage on SMA and accumulation rates. The investigations have been conducted over five years at 24 communal systems in Indonesia, India and South Africa.